1. Field of the Invention
This invention relates to digital radio systems and more specifically to transmitting, receiving and demodulating digital information in a Time Division Multiple Access (TDMA) radio system.
2. Description of Related Art
In time division multiple access (TDMA) radio systems, base units communicate with a plurality of mobile radio units. Each mobile radio is allocated a frequency channel and a time slot within that channel to communicate with the base unit. A transmitter of one of the communicating units transmits predetermined reference symbols embedded at periodic intervals in a continuous radio-frequency (RF) signal allowing a receiver to synchronize with these reference symbols and separate the RF signal into frames and time slots. The receiver searches for the reference symbols by comparing the received signal to an internal set of reference symbols. Typically, a mobile receiver disregards all information except the slot which is intended for it.
Channel impairments such as RF interferences, radio signal "fades" and other RF distortion may cause improper reception at the receiving unit even if the impairment is transient in nature.
The improper reception translates to an increased bit error rate (BER) in the decoded digital information. In addition to the above-mentioned impairments, the mobile radio receiver has to contend with radio signal reflected off of physical objects creating echoes known as multipath propagation. To combat multipath propagation problems, an equalizer is required in the radio receiver.
Conventional radio receivers decode a received RF signal by sampling the signal into a slot of samples and decoding the slot in a forward direction from the first received sample, the start of the slot to the last received sample, being from the end of the slot. This decoding is in a forward sense. Alternatively, the decoder may begin decoding at the end of the slot and works its way to the beginning of the slot, referred to as reverse decoding or decoding in a reverse sense. These schemes may be referred to as single sided decoding schemes since they decode in a single direction being either the forward or reverse sense. Single sided decoding schemes are prone to error propagation since errors in decoded samples are incorporated in calculations involving decoding of subsequent samples. Therefore, a single error at the beginning of a slot is propagated resulting in multiple errors over the slot. This is common in implementing automatic frequency control (AFC), timing recovery, channel estimation, etc.
The aforementioned Bi-directional Demodulation application by Paul W. Dent and Sandeep Chennakeshu describes a method of decoding a slot in a forward direction and a backward direction and choosing the direction which gives the best result. This does not require any additional reference information.
Currently there is a need for a system for demodulating TDMA information having greater fade resistance and superior performance than existing systems.